1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon coking operations in which one or more gaseous streams comprising oxygen is added to the coker transfer line which carries feedstock to the coker, and an improvement involving adding one or more gaseous streams comprising oxygen to a coker into which sludge is also being added.
2. General Background
Coking operations in most modern refineries produce solid coke, and vapor products from heavy residual oil feedstocks which are fed to the coking process. The coking process can be either a delayed coking operation or a fluidized coking operation.
In fluid coking, a feedstock contacts a fluidized bed of coke particles maintained at a sufficiently high temperature to effect conversion of the feed into solid coke particles and lighter liquid and vapor materials which are recovered from the fluidized bed. Part of the solid coke formed in this operation is passed into a separate gasifier vessel where it is burned to produce additional heat. This heat is recycled back into the fluid bed of coke particles in the reaction section through higher temperature coke particles which provide heat to help maintain process operations.
In the more usual application of the coking process, a delayed coking drum is used. A heavy residual oil is heated in a furnace, passed through a transfer line and then into the coking drum. In the coking drum, which is typically an elongated vessel, the residual feedstock is thermally decomposed to a heavy tar or pitch material which further decomposes with time into solid coke and vapor materials. The vapor materials formed during the coking reaction are recovered from the delayed coking drum and a solid coke material is left behind.
The vapor products are removed from the top of the coke drum through a coke drum vapor outlet and passed through an elongated coke drum overhead line which is connected to a fractionator, often called a combination tower. In the combination tower, gaseous and liquid products are recovered for further use in the refinery.
After a period of time the feed to the coke drum is stopped and routed to another drum and the coke laden drum is then purged of vapors, cooled and opened so that the solid coke inside the drum can be removed.
In operating a coking process, the refiner generally aims to minimize coke production and maximize liquid products, since the liquid is more easily converted into gasoline or other materials having higher values than the solid coke material.
The delayed coking furnace outlet temperature is controlled between from about 870.degree. to 950.degree. F. Higher temperatures reduce the solid coke yield and increase the more valuable liquid product yield but may cause rapid coking in the furnace tubes and shortened on-stream time for the process. Lower transfer line temperatures produce soft coke, higher coke yield, and lower liquid yield but permit long on-stream time for the process.
The coke formation reactions are essentially endothermic with the temperature dropping to 780.degree. to 900.degree. F., more usually to 810.degree. to 880.degree. F. in the coke drum. Coke drum pressures are maintained in the range generally from about 10 to 70 psig.
The transfer line connects the coker furnace to the coke drum, and the temperature of the coker feedstock passing through the transfer line is typically called the transfer line temperature. Raising the temperature of the transfer line increases yields of valuable liquid products while reducing the yield of solid coke. Since a primary object of delayed coking processes in refinery environments is to maximize the production of valuable liquid products from the residual feeds, maximizing the liquid yield while minimizing the solid coke yields is desirable.
To maximize the transfer line temperatures, various methods have been used to increase the coker feed temperatures while reducing or minimizing any adverse effects accompanying these higher temperatures. Adding hot coke particles to the delayed coker feed has been disclosed. Adding oxygen-containing solids to increase transfer line temperature through oxidation of the feed passed into the coking drum is known. Additional methods for increasing transfer line temperatures include combustion of part of the feed or coke produced in the delayed coker in a separate combuster which is heat exchanged with the coker feed.
U.S. Pat. No. 2,412,879 discloses a process in which a cellulosic material such as sawdust is added to delayed coker feed to reduce the amount of solid coke produced from the feedstock and to produce an easily crushable and porous solid coke material. The cellulosic material is converted at least partially to charcoal indicating that some oxidation of the sawdust material occurs before entry of the sawdust-feed mixture into the delayed coking drum.
U.S. Pat. No. 4,096,097 similarly teaches a process of producing high quality coke in a delayed coking process by adding an effective amount of an oxygen-containing carbonaceous material which decomposes at the high temperatures of the feed passing into the delayed coking drum. As disclosed in this patent, the oxygen content of the carbonaceous additive should be within the range from about 5 to 50 weight percent and usually no higher than 60 weight percent of the oxygen-containing material added to the feed. The carbonaceous materials which are taught to be effective include coal, lignite, and other materials such as sugar beet waste, sawdust, and other cellulosic wastes. It is speculated in this patent that the decomposition of the oxygen-containing molecules at the coking temperatures in the coking drum effect the release of heat. Water is also produced which promotes increased liquid yields and a more porous structure of the solid coke material.
U.S. Pat. No. 4,302,324 also relates to an improved delayed coking process in which hot coke particles are added to the heated coker feedstock to raise its temperature by at least 50.degree. F. The coke produced in this process is lower in volatiles and has improved mechanical strength, and the yield of liquid product is increased.
Another process involves coking hydrocarbon oils by contacting a feed with free oxygen in the presence of an aqueous liquid which is maintained at least partially in the liquid phase to produce high quality coke and increase yields of liquid products from the coking reaction. This process is exemplified in U.S. Pat. Nos. 4,370,223 and 4,428,828. Sometimes the entire heat requirements for the process can be provided by the oxidation of the heavy hydrocarbon feed in the aqueous system with free oxygen.
Another process in which oxygen reacts with a residual feed is asphalt blowing. This process is exemplified in U.S. Pat. No. 3,960,704 in which isotropic petroleum coke is produced from a residual feedstock by blowing the feedstock with air until it has a softening temperature between from about 49.degree. to 116.degree. C. and subjecting the blown residuum to a delayed coking process.
The fluid bed coking art is replete with patents in which air or oxygen is added to a fluidized coking process to enhance fluid coke properties and decrease the need for external heat addition to the process. In particular, U.S. Pat. Nos. 2,537,153, 3,264,210, 3,347,781, 3,443,908, and 3,522,170 discuss various methods for using oxygen either directly injected into a fluid bed of coke or combusting a part of the fluid coke with the oxygen to supply additional heat to the fluid bed process.
One of the advantages associated with oxygen addition to the coker transfer line, as claimed by Applicants, is a decrease in the amount of coke produced with an increase in the liquid product produced in the delayed coking zone because the feed to the coking drum is at sufficiently high temperature to encourage these results. The additional heat generated by partial feedstock combustion in the transfer line results in increased temperatures in the transfer line rather than in the furnace. This reduces the risk of coking in the furnace tubes resulting in a higher operating factor for the delayed coker furnace
Since the oxygen addition is to a feed having a temperature above about 800.degree. F., it is important that oxygen addition be regulated by careful positioning of the oxygen addition stream or streams. At temperatures above 800.degree. F. oxidation of the feed can occur very rapidly and if the oxygen-containing stream is not properly controlled, high temperature excursions can result.
Sludge production from a typical refinery or petrochemical plant can come from many sources including API separator bottoms, slop oil, emulsions, storage tank bottoms, sludge from heat exchangers, oily waste, MEA reclaimer sludges, and other waste materials produced in the refinery. The typical sludge will contain solids, which may be organic, inorganic or combinations of both, oil, liquid and aqueous materials.
In most refinery or petrochemical operations the sludge is often sent to a separator zone for gross removal of water and hydrocarbons after which the water and concentrated hydrocarbons and solids can be individually treated by landfarming or further biological or other known waste treatment means.
In U.S. Pat. No. 4,552,649 (U.S. Class 208/127), an improved fluid coking process is described where an aqueous sludge which comprises organic waste material is added to a quench elutriator to cool the coke in the elutriator and convert at least a portion of the organic waste to vaporous compounds which can be recycled to the fluid coking heating zone to increase the temperature of the fluid coke particles therein.
In the delayed coking process, sludges have been disposed of in various manners.
In U.S. Pat. No. 3,917,564 (U.S. Class 208/131) sludges or other organic by-products are added to a delayed coking drum during a water quenching step after feed to the coke drum has been stopped and the coke drum has been steamed to remove hydrocarbon vapors. The quenching step cools the hot coke within the coke drum to a temperature that allows the coke to be safely removed from the coking drum when it is opened to the atmosphere.
The sludge is added along with the quench water and contacts the solid coke in the coke drum at conditions which allow the vaporization of the water contained in the sludge. The organic and solid component of the sludge is left behind through deposition on the coke and removed from the coke drum as part of the solid coke product.
U.S. Pat. No. 4,666,585 (U.S. Class 208/131) relates to the disposal of sludge in a delayed coking process by adding sludge to the coker feedstock and subjecting the feedstock and sludge mixture to delayed coking conditions.
U.S. Pat. No. 2,043,646 (U.S. Class 202/16) discloses a process for the conversion of acid sludge into sulfur dioxide, hydrocarbons and coke in a two-step procedure comprising passing sludge into a kiln to produce semi-coke and then passing the semi-coke into a coke drum for conversion into coke product.
U.S. Pat. No. 1,973,913 (U.S. Class 202/37), coke which has been removed from a coking oven or coking drum is quenched with polluted wastewater which contains tar acids. After quenching the tar acids remain on the coke and the aqueous materials associated with these acids is vaporized.
U.S. Pat. No. 4,404,092 (U.S. Class 208/131) discloses a process for increasing the liquid yield of a delayed coking process by controlling the temperature of the vaporous space above the mass of coke in the coke drum by injecting a quenching liquid into the vapor phase within the delayed coking drum. The patent teaches that large amounts of liquid should be added to the vapor space within a delayed coking drum (about 9 percent by weight of the feed).
U.S. Pat. No. 2,093,588 (U.S. Class 196/61) discloses a process for delayed coking in which liquid materials such as hydrocarbons or water are passed into the vapor portion of a delayed coking zone. This patent teaches a process very similar if not identical to that disclosed in U.S. Pat. No. 4,404,092 described above.
Copending application U.S.S.N. 285,111 (Docket No. 26,878) filed concurrently with this application, claims a process for adding sludge to a coking zone. The heat requirements for evaporating the sludge and converting it to non-toxic material can be supplemented by the heat generated through gaseous oxygen addition as described in the present application.
Another aspect of the present invention is to combine oxygen addition to a coking zone as described in the present application with sludge addition to the coking zone as described in the above copending application to provide an improved sludge addition process.
When oxygen is added to a coking zone to which sludge is being added, it is preferable to add the oxygen to the transfer line where it can mix with hot feed. However, in such cases the oxygen could be added to the coking zone at other locations such as in the coke drum or along with the sludge.
Sludge addition may take place at any convenient location in the coke drum. The preferred locations, however, are in the feed or in the vapor section of the coke drum. In the latter case, sludge is generally added as a separate stream, at conditions to effect contact of the sludge with the vapor products within the coke drum and vaporization of at least a portion of the sludge while oxygen is preferably added to the transfer line. In a preferred instance, all of the aqueous portion of the sludge is vaporized and some of the hydrocarbon in the sludge is converted to coke while oxygen is added to the transfer line.
The improved sludge addition process also can eliminate a major concern of having to dispose of potentially hazardous materials by breaking them down into relatively harmless materials which themselves can be further converted into useful refinery products such as gasoline or other refinery products. This also eliminates the need for land farms or other waste disposal methods which can add considerable expense to refinery operations.